Map-Reduce

Map-Reduce Examples

In the mongo shell, the db.collection.mapReduce() method is a wrapper around the mapReduce command. The following examples use the db.collection.mapReduce() method:

Consider the following map-reduce operations on a collection orders that contains documents of the following prototype:

{
     _id: ObjectId("50a8240b927d5d8b5891743c"),
     cust_id: "abc123",
     ord_date: new Date("Oct 04, 2012"),
     status: 'A',
     price: 25,
     items: [ { sku: "mmm", qty: 5, price: 2.5 },
              { sku: "nnn", qty: 5, price: 2.5 } ]
}

Return the Total Price Per Customer

Perform map-reduce operation on the orders collection to group by the cust_id, and for each cust_id, calculate the sum of theprice for each cust_id:

1. Define the map function to process each input document:
   • In the function, this refers to the document that the map-reduce operation is processing.
   • The function maps the price to the cust_id for each document and emits the cust_id and price pair.

   var mapFunction1 = function() {
                          emit(this.cust_id, this.price);
                      };
   

2. Define the corresponding reduce function with two arguments keyCustId and valuesPrices:
   • The valuesPrices is an array whose elements are the price values emitted by the map function and grouped bykeyCustId.
   • The function reduces the valuesPrice array to the sum of its elements.

   var reduceFunction1 = function(keyCustId, valuesPrices) {
                             return Array.sum(valuesPrices);
                         };
   

3. Perform the map-reduce on all documents in the orders collection using the mapFunction1 map function and thereduceFunction1 reduce function.

   db.orders.mapReduce(
                        mapFunction1,
                        reduceFunction1,
                        { out: "map_reduce_example" }
                      )
   

   This operation outputs the results to a collection named map_reduce_example. If the map_reduce_example collection already exists, the operation will replace the contents with the results of this map-reduce operation:

Calculate Order and Total Quantity with Average Quantity Per Item

In this example you will perform a map-reduce operation on the orders collection, for all documents that have an ord_date value greater than 01/01/2012. The operation groups by the item.sku field, and for each sku calculates the number of orders and the total quantity ordered. The operation concludes by calculating the average quantity per order for each sku value:

1. Define the map function to process each input document:
   • In the function, this refers to the document that the map-reduce operation is processing.
   • For each item, the function associates the sku with a new object value that contains the count of 1 and the item qty for the order and emits the sku and value pair.

   var mapFunction2 = function() {
                          for (var idx = 0; idx < this.items.length; idx++) {
                              var key = this.items[idx].sku;
                              var value = {
                                            count: 1,
                                            qty: this.items[idx].qty
                                          };
                              emit(key, value);
                          }
                       };
   

2. Define the corresponding reduce function with two arguments keySKU and countObjVals:
   • countObjVals is an array whose elements are the objects mapped to the grouped keySKU values passed by map function to the reducer function.
   • The function reduces the countObjVals array to a single object reducedValue that contains the count and the qtyfields.
   • In reducedVal, the count field contains the sum of the count fields from the individual array elements, and the qty field contains the sum of the qty fields from the individual array elements.

   var reduceFunction2 = function(keySKU, countObjVals) {
                        reducedVal = { count: 0, qty: 0 };
   
                        for (var idx = 0; idx < countObjVals.length; idx++) {
                            reducedVal.count += countObjVals[idx].count;
                            reducedVal.qty += countObjVals[idx].qty;
                        }
   
                        return reducedVal;
                     };
   

3. Define a finalize function with two arguments key and reducedVal. The function modifies the reducedVal object to add a computed field named avg and returns the modified object:

   var finalizeFunction2 = function (key, reducedVal) {
   
                          reducedVal.avg = reducedVal.qty/reducedVal.count;
   
                          return reducedVal;
   
                       };
   

4. Perform the map-reduce operation on the orders collection using the mapFunction2, reduceFunction2, andfinalizeFunction2 functions.

   db.orders.mapReduce( mapFunction2,
                        reduceFunction2,
                        {
                          out: { merge: "map_reduce_example" },
                          query: { ord_date:
                                     { $gt: new Date('01/01/2012') }
                                 },
                          finalize: finalizeFunction2
                        }
                      )
   

   This operation uses the query field to select only those documents with ord_date greater than new Date(01/01/2012). Then it output the results to a collection map_reduce_example. If the map_reduce_example collection already exists, the operation will merge the existing contents with the results of this map-reduce operation.